Credential Stuffing in Buffalo with Bearer Tokens

Credential stuffing is an automated attack technique where previously breached username and password pairs are reused to gain unauthorized access to accounts. When this pattern intersects with Buffalo applications that rely on Bearer tokens for authentication, the risk surface expands in three dimensions: the application layer, the token handling layer, and the session validation layer.

Buffalo is a web framework for Go that encourages rapid development with sensible defaults, but like many frameworks it does not inherently prevent token misuse when credentials are compromised. In a typical Buffalo service, developers issue a Bearer token (often a JWT) after a successful login and store it in an Authorization header for subsequent requests. If an attacker runs a credential stuffing campaign against the login endpoint, they are not directly targeting the token issuance logic but are probing for valid accounts. Successful authentication returns a valid Bearer token, which the attacker can then replay to access protected resources.

The exposure becomes tangible when token lifecycle and scope controls are weak. For example, if tokens have long expiration times, are not bound to a particular client context, or lack proper revocation mechanisms, a token obtained via credential stuffing remains usable for an extended period. Additionally, if the Buffalo application does not enforce strict token validation—such as verifying the issuer, audience, and signature rigorously—an attacker can use a token from a breached external system if the same credentials were reused across services.

In the context of the 12 security checks run by middleBrick, credential stuffing intersects with Authentication, BOLA/IDOR, and Rate Limiting. An unauthenticated scan can detect whether the login endpoint is rate-limited, whether tokens are issued after failed attempts, and whether authorization checks are consistently applied across endpoints that accept Bearer tokens. Without proper rate limiting or account lockout strategies, the Buffalo application becomes a conduit for credential stuffing, and Bearer tokens amplify the impact by providing a portable credential that can be reused across systems.

Real-world attack patterns mirror this scenario: an attacker obtains a list of breached credentials, submits them against the Buffalo login route, captures the returned token, and then probes for IDOR or BOLA vulnerabilities to escalate access. middleBrick’s checks for Authentication and BOLA/IDOR help surface these weaknesses by correlating runtime behavior with the OpenAPI specification, identifying endpoints where tokens are accepted without sufficient context validation.

Remediation focuses on hardening how Bearer tokens are issued, validated, and scoped within a Buffalo application. The goal is to reduce the effectiveness of credential stuffing by ensuring that token usage is tightly coupled with the authentication event and constrained by validation and lifecycle controls.

1. Enforce strict token validation
Always validate the token’s signature, issuer (iss), audience (aud), and expiration (exp). Use a well-audited JWT library rather than manual parsing. In Buffalo, you can implement a middleware that checks the Authorization header before routing requests.

package middleware

import (
	"github.com/golang-jwt/jwt/v5"
	"net/http"
)

func BearerTokenMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		auth := r.Header.Get("Authorization")
		if auth == "" {
			http.Error(w, `{"error":"authorization header required`", http.StatusUnauthorized)
			return
		}

		tokenString := auth[len("Bearer "):]
		token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
			// TODO: use the appropriate key, e.g., RSA public key or secret
			return []byte("your-256-bit-secret"), nil
		})
		if err != nil || !token.Valid {
			http.Error(w, `{"error":"invalid token`", http.StatusUnauthorized)
			return
		}

		// Optionally verify claims like iss, aud here
		next.ServeHTTP(w, r)
	})
}

2. Bind tokens to authentication context
Avoid issuing long-lived tokens for high-risk actions. Instead, tie token issuance to the specific authentication event and include minimal claims. For example, include the user ID and a session identifier, but avoid embedding permissions that can be escalated via IDOR.

claims := jwt.MapClaims{
	"sub": user.ID,
	"sid": sessionID,
	"iat": time.Now().Unix(),
	"exp": time.Now().Add(time.Hour * 1).Unix(),
	"scope": "read write",
}
token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
tokenString, err := token.SignedString([]byte("your-256-bit-secret"))
if err != nil {
	// handle error
}

3. Implement rate limiting on authentication endpoints
Even though this is not token-specific, it directly mitigates credential stuffing. Use a sliding window or token bucket algorithm at the Buffalo route level to limit login attempts per IP or per user identifier.

// Pseudo-implementation outline
// In routes.go
app.Post("/login", func(c buffalo.Context) error {
	ip := c.Request().RemoteAddr
	if !rateLimiter.Allow(ip) {
		return c.Render(http.StatusTooManyRequests, r.JSON(map[string]string{"error": "rate limit exceeded"}))
	}
	// proceed with authentication and token issuance
	return nil
})

4. Apply principle of least privilege and avoid excessive agency
When designing APIs that accept Bearer tokens, scope tokens to the minimum required operations. Do not issue tokens with broad administrative scopes as a default. If your application uses patterns that could resemble excessive agency (e.g., dynamic function calls based on token claims), validate and restrict these paths explicitly.

5. Revocation and short expiration
Prefer short-lived tokens and implement a revocation list or use token versioning (e.g., a jti claim tracked in a store). This ensures that even if a token is harvested via credential stuffing, its usability window is limited.

// Example token with JTI
claims["jti"] = uuid.NewString()
claims["exp"] = time.Now().Add(time.Minute * 15).Unix()

By combining these practices—strict validation, contextual binding, rate limiting, least privilege, and short-lived tokens—you reduce the risk that a Bearer token issued after a successful credential stuffing attack leads to widespread compromise in a Buffalo application.